Silicon photonics relate to photonic systems, where silicon is used as medium for light propagation because of the material's low loss. Silicon photonics makes use of well-established silicon manufacturing principles exploited in complementary metal-oxide semiconductor (CMOS) electronics. The features are usually patterned into micro-photonic components with sub-micron precision (to operate in the infrared). Silicon on insulator (SOI) is typically used as a material of choice. The fabrication of silicon photonic devices can otherwise involve known semiconductor fabrication techniques; since silicon is already used as a substrate of choice for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single chip.
To meet the requirements of future computing systems, high speed and energy efficient alternatives to on-chip electrical interconnects are needed. Integrated optics, in particular silicon photonics, meet such requirements. Integrated optical interconnects with compatible light sources are needed for mass-fabrication of low-cost, high-performance CMOS-based chips. Due to the indirect band gap of silicon, no Si-based light source is available. Efficient light sources are typically based on III-V semiconductors which are heterogeneously or hybrid integrated on a Si photonics platform.
The most promising approaches to date resort to bonding a full epitaxial III-V-based gain layer stack, or a thin seed layer, which can be subject to successive re-growth, on top of the silicon photonic waveguides. In either case, to measure, characterize and evaluate the laser structures or light sources, special test structures are needed in order to obtain insight into key device parameters. One of the key parameter is the optical gain. The computation of the gain is not very precise and thus the measurement of the gain is needed, to model gain layers and thus to optimize the active optical devices, such as lasers or optical amplifiers. However, the gain measurement is very challenging for on-chip applications such as silicon photonics. The reason is that existing techniques and methods involve cleaving facets or require numerous devices, which is unattractive for implementation in on-chip applications, as this method is time consuming, destructive and/or due to the large real-estate consumption. Moreover, such techniques can require high resolution spectrometers in order to resolve the oscillations in the output power spectrum. In addition, the majority of the existing concepts can only extract the optical gain below or around threshold, which are to common modes of operation for a laser device. Thus, the known standard techniques do not make it possible to characterize gain properties at the operation condition of the gain material under realistic pumping conditions as used in a light emitting device of interest (i.e., at pumping current densities being equivalent to the operation conditions of the targeted laser).
In conclusion, no appropriate device exists, which allows to simply measure the gain of an on-chip optical device, i.e., without fabricating numerous devices, having to cleave the device or polish its facets, to apply multiple contact to the gain measurement structure, or to measure the gain accurately above threshold under realistic pumping/operating conditions.